The term "molecular sieve" refers to a wide variety of positive ion containing crystalline materials of both natural and synthetic varieties which exhibit the property of acting as sieves on a molecular scale. A major class of molecular sieves are crystalline aluminosilicates, although other crystalline materials are included in the broad definition. Examples of such other crystalline materials include coal, special active carbons, porous glass, microporous beryllium oxide powders, and layer silicates modified by exchange with organic cations. See, D. W. Breck, "Zeolite Molecular Sieves: Structure, Chemistry, and Use", John Wiley & Sons, 1974.
Zeolites are crystalline, hydrated, framework aluminosilicates which are based on a three-dimensional network of AlO.sub.4 and SiO.sub.4 tetrahedra linked to each other by sharing all of the oxygens.
Zeolites may be represented by the empirical formula EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
wherein, x is generally equal to or greater than 2 since AlO.sub.4 tetrahedra are joined only to SiO.sub.4 tetrahedra, and n is the cation valence. The framework contains channels and interconnected voids which are occupied by the cation, M, and water molecules. The cations may be mobile and exchangeable, to varying degrees, by other cations. Intracrystalline zeolitic water in many zeolites is removed continuously and reversibly. In many other zeolites, mineral and synthetic, cation exchange or dehydration may produce structural changes in the framework. Ammonium and alkylammonium cations may be incorporated in synthetic zeolites, e.g., NH.sub.4, CH.sub.3 NH.sub.3, (CH.sub.3).sub.2 NH.sub.2, (CH.sub.3).sub.3 NH, and (CH.sub.3).sub.4 N. In some synthetic zeolites, aluminum cations may be substituted by gallium ions and silicon ions by germanium or phosphorus ions. The latter necessitates a modification of the structural formula.
The structural formula of a zeolite is best expressed for the crystallographic unit cell as: M.sub.x/n [(AlO.sub.2).sub.x (SiO.sub.2).sub.y ].wH.sub.2 O where M is the cation of valence n, w is the number of water molecules and the ratio y/x usually has values of 1-100 depending upon the structure. The sum (x+y) is the total number of tetrahedra in the unit cell. The complex within the [] represents the framework composition.
Mordenite is a form of zeolite that occurs naturally and has been prepared synthetically. A typical formula for mordenite in terms of oxides is as follows: EQU Na.sub.2 O:Al.sub.2 O.sub.3 :9-10SiO.sub.2 :6H.sub.2 O
The framework structure typically consists of chains of 5 rings cross linked by 4 rings. The chains consist of 5 rings of SiO.sub.4 tetrahedra and single AlO.sub.4 tetrahedra. Mordenite can be in the form of small port or large port and is characterized by a relatively high degree of thermal stability. See, D. W. Breck, Supra, which is incorporated herein for its teaching with respect to mordenite.
The term "high silica mordenite" is used herein to refer to a mordenite structure characterized by a Si/Al mole ratio in excess of 5. The literature of molecular sieves suggests that it is not possible to make high silica mordenite by a method that is direct and efficient.
Synthesis directing agents are agents which when added to a reaction mixture effect a different crystalline structure than that which would occur but for the addition of such agent, or result in the formation of a crystalline structure wherein an amorphous mixture would otherwise remain. Examples of conventional agents that are currently in use include ammonium and tetrapropyl ammonium ions, and alkyl phosphonium and alkyl arsonium ions. See, L. D. Rollman, "Templates in Zeolite Crystallization", Adv. Chem. Ser., 173, 387 (1979).
U.S. Pat. No. 4,018,870 discloses the use of certain nitrogen containing dyes including methyl violet and methylene blue in the synthesis of zeolites. This patent indicates that these dyes are used to suppress the formation of unwanted zeolites during the manufacture of more desirable products.
It would be advantageous to provide a method for making high silica mordenite that is both direct and efficient.